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0 Journal of Manufacturing Systems w Vol 2uNo 1 d 2002 Trends and Perspectives Computer Aided Fixture Design Using Information Intensive Function Models in the Development of Automated Fixture Design Systems J Cecil Assistant Professor Virtual Enterprise Engineering Lab VEEL Industrial Engineering Dept New Mexico State University Las Cruces New Mexico USA Abstract In this paper the development of an Information Intensive Function Model IIFM of fixture design activities is dis cussed This function model was developed using the IDEF 0 Integrated Definition methodology and was built primarily after interacting with expert fixture designers In addition knowledge from fixture design tool design and metalcutting handbooks and from journal papers was used to lend a holis tic flavor to the developed IDEF 0 model An IDEF 0 model ing software tool AI0 Win was used to build this model at various levels of decomposition This paper also discusses the importance of IlFMs in the current and future design of Computer Aided Fixture Design CAFD systems Keywords Fixture Design Computer Aided Fixture Design Function Modeling CAD CAM Information Intensive Function Models Introduction Fixture design is a complex task that has been viewed by industrial practitioners and researchers as a vital link between design and manufacturing activ ities Sakurai 1990 Senthil Kumar Subramaniam Seow 1997 Computer Aided Fixture Design CAFD is a crucial task whose realization will enable the integrated accomplishment of computer aided design CAD and computer aided manufac turing CAM activities in modern manufacturing environments There have been numerous efforts to automate fixture design activities and develop CAFD methods for rotational and prismatic parts manufacturing see references l 22 24 39 41 42 45 46 However there has been a lack of effective CAFD techniques to fill the void between CAD and CAM and automated fixture design techniques con tinue to be the focus of manufacturing research efforts There is a need to better understand the com plex tasks and subtasks within the fixture design 58 activity by developing a function model of overall activities Such a model would not only help the understanding of the complex functions and task accomplishments within fixture design but would result in a model of what was being accomplished by the various functions In general a function model can be described as a representation of activities within a target focus area or system and the rela tionships between them A function model can also enable understanding of the impact and role of vari ous information inputs and controls in the accom plishment of activities Past efforts have not emphasized the use of for mal models or representations as a basis for design ing and developing CAFD systems The research activity discussed here explores the role of informa tion intensive function models IIFM which can be a valuable tool in obtaining a better understanding of complex activities and tasks in a domain such as fix ture design Further these models provide a struc tured basis of communication to concurrent engi neering CE teams who will be impacted by and who impact the activities within fixture design Function models can also be used as a requirements definition map by software engineers who support the implementation of software systems that accom plish CAFD A formally structured model can more effectively convey the complex relationships and dependencies among subactivities Other engineers such as product designers or machinists involved in the life cycle of a product and who are not famil iar with fixture design activities can get a better understanding of the interrelationships and the need for feedback and timely communication among their respective CE teams before an infeasible design idea progresses toward other downstream Journal of Manufacturing Systems Vol 21 No 1 2002 activities The power of IIFMs lies in their ability to describe what can or will be accomplished by a sys tem human team members and other resources IIFMs explicitly capture intricate relationships between information sources bottleneck oriented constraints for example availability of data or some other internal attribute and the software or hard ware mechanisms that work together to accomplish a given activity In today s information technology oriented manufacturing environment the design of manufacturing related software systems must be accomplished taking into consideration such infor mation attributes that are captured in IIFMs Fixture design is a complex task and understanding the relationships explicitly and building an IIFM of these attributes lays the foundation for implement ing effective CAFD approaches It also provides a common language for reasoning a specific approach enables CE team members to understand and communicate potential problems in informa tion availability more effectively and proposes ways to accomplish a core set of activities in a more integrated manner Using the function model of fix ture design activities a preliminary version of a CAFD system called TAMIL Towards A Manufacturing Integration Link has been devel oped Cecil 2001 The IIFM provided a fundamen tal basis to understand and map complex relation ships study information flow and then design and build a system to automate fixture design activities IDEF 0 methodology was used to build the IIFM The IDEF 0 model resulting from the use of this methodology provided a foundation for understanding the complex activities in fixture design and facilitated the definition of activities and information surround ing these functions In other initiatives the IDEF 0 method has been used as a powertil modeling tool for analysis specification and design of manufacturing and other systems Cecil 1995 Mayer Keen Blinn 1990 The four basic attributes modeled for each activity and subtasks include inputs controls out puts and mechanisms termed ICOMs see Figure I The IDEF 0 method includes a graphical language that enables the user to describe the activities being modeled in a powerful hierarchical manner The root or highest level in this hierarchical structure has lower level or children nodes that detail the fUnctiona activity being modeled A traversal of this hierarchy unfolds the details of the children activities and the objects that form the relations between these activities INPUT CONTROL MECHANISM OUTPUT Figure I ICOM Attributes Illustrated in an IDEF 0 Model Function Model Development and Descriptions The IDEF 0 function model discussed was built after studying the accomplishment of fixture design activities by expert fixture designers and process planners Knowledge and information from fixture design tool design and metalcutting handbooks and from journal papers has also been used to develop a more holistic IDEF 0 model Additional discussion of the information represented in an IDEF 0 model is provided in the section entitled IDEF 0 Function Model of Fixture Design Domain experts included a process planner expert A a tool engineer expert B and an engi neering instructor expert C Expert A works in a machine shop and has more than 20 years of experi ence performing process planning fixture design and machining in a job shop which produces both rotational and prismatic parts Expert B is a tool engineer in the automobile industry with a decade of industrial experience who designs jigs and fixtures for parts used in th e production of cars and trucks Expert C is a mechanical engineering instructor who teaches fixture design related courses and had pre viously worked as a fixture designer in the automo bile industry Verbal Protocol Analysis was used as the method of acquiring detailed knowledge from the domain experts After being given a product design and designing the fixture each expert pro vided explanations of a list of possible designs or conclusions After the experts completed the nec essary explanations the model developer reviewed the description of the protocol or steps used to arrive at the fixture designs and added more details if necessary Cecil 2000 59 Journal of Manufacturing Systems Vol 21rNo 1 2002 Another protocol used in knowledge acquisition for the fixture design activity was blind problem dis cussion Tuthill 1990 Fixture designs from another knowledge expert as well as from books and papers were used in the discussion Each fixture design expert was presented with a part design design A and asked to design a fixture for that part design Each expert s organization of knowledge the hypotheses considered the influence of various fac tors during analysis and the determination of the overall detailed design were recorded This recorded approach was compared with the approach described by other fixture design experts for the same part design By comparing and contrasting the approaches of these fixture designers a better understanding of the fixture design activity was obtained This technique was repeated with approaches detailed in books and manuals Numerous questions were asked to clarify why cer tain tasks were performed and what role specific factors or inputs played in the fixture design The intent was to obtain an understanding of analysis cri teria in fixture design Typical questions explored by the model developer included what types of infor mation were required to determine problem bound aries what level of detail in the solution or design was required how to decompose a problem into sub problems and what types of analytical and heuristic rules were typically used or required to validate the final design solution without having to build an actual fixture of the developed fixture design The domain experts conclusions were analyzed further by posing what if scenarios to understand more about the problem solution A follow up post session discussion helped explore each factor and attribute that seemed to influence development of the problem solution This discussion helped estab lish individual contributions of the various inputs or factors such as tolerances feature attributes etc to the overall fixture design activity IDEF 0 Function Model of Fixture Design In this section the IDEF 0 model of fixture design is presented and a brief discussion of each activity performed in fixture design is provided In general an IDEF 0 model includes a summary purpose viewpoint context decomposition diagrams descriptions of the modeled activities and a glossary Fixture dyign principles concepts Product desiqn data Initial raw workpiece specifications Tentative machining sequences Develop fixture designs t Fixture designer Figure 2 A O Level Diagram of Fixture Design Activity of the ICOM attributes captured in the model In this paper the top level and decomposition diagrams are illustrated in Figures 2 3 4 6 8 9 and 13 The purpose of building this IDEF 0 model was to provide a better understanding of the various activities and subactivities and tasks performed within fixture design and capture the complex inter relationships among these activities and their decompositions This function model was used to build an automated fixture design system to serve as a link between CAD and CAM activities The mod eling viewpoint was from an industrial engineer with a strong background in both design and manu facturing who has performed fixture design activi ties and is familiar with fundamental fixture design tasks The context for building any IDEF 0 function model is provided in a diagram referred to as a con text diagram shown in Figure 1 The specific con text level diagram for the modeled fixture design activities is shown in Figure 2 For the fixture design activities inputs assumed to be available to the fix ture designer include product design information such as design drawings CAD models tolerance specifications etc and workpiece material Inputs can be real objects or data needed to perform a l the fixture designer identifies the tolerance planes or datum reference planes for each feature This activity is a precursor to identifying the datum or face f2 Figure 5 Example of a Part Design Decomposition of Activity A2 Analyze machining requirements reference planes in each setup which is per Under the category Analyze Machining formed later Figure 5 illustrates an example of a Requirements Figure 6 the first activity involves part design with tolerances units in mm and two identifying machining requirements A2 1 The fix features step and a blind hole The datum planes ture designer studies the machining requirements of given are faces f5 f7 and fl each feature in the given part design Any machining Ml Decomposition of Analyze Machining Requirements Activity A2 63 Journal of Manufacturing Systems Vol 21mo 1 2002 constraint that may influence the design of the fix ture is identified The machining constraints influ encing fixture design depend on the type of feature being machined An example of this would be whether the machining of a feature such as a through hole or slot influences the design of a sup port or locator Machining a through hole requires that care be taken while designing a support A key aspect to be addressed is ensuring that the machine cutting tool such as a drill is able to drill through the bottom face and clear the bottom face in ref erence This would require that the support device used be designed to provide a relief for the drill tool to clear the bottom face while machining the through hole in reference The machining of a slot or any other feature that straddles two faces or occurs across two faces also needs to be studied Machining of slots necessitates a side height clear ance to allow the milling cutter to machine the entrance of the slot The face possessing the entrance of a slot can be used as a locating or addi tional locating surface as long as the locating device does not obstruct the machining of the slot Figure 7 Issues related to the machining of features are identified in this activity and are useful when the fixture designer performs detailed locator support and clamp design in the later stages The next activity Identify chip removal require ments as indicated in Figure 6 is closely related to activity A21 The accumulation of chips during machining of features such as through holes needs to be addressed The type of machine tool used also influences the chip accumulation problem For example if a machine tool such as a gun drill is used chip accumulation is not a problem because a gun drill collects the chips with the used coolant during drilling and disposes them using a suction mechanism When other drill tools are used the fix ture designer has to incorporate a relief design in the support device to minimize the accumulation of chips during machining Another activity relates to grouping possible operations A23 in Figure 6 The major machining constraint addressed by the fixture designer is in the generation of setups the goal in this activity is to enable the machining of as many features as possi ble in a setup The major constraint is the capability of the machines available on the shop floor tenta tively specified by the process planner see Figure 2 Various setups to machine the features on the Horizontal Dimensions of locator should allow for machine tool clearance to machine the slot Figure 7 Analyzing Machining Requirements given part design are generated based on the direc tion of machining the process sequence and the machining capabilities of available machines on the shop floor The fixture designer identifies the machining directions to produce each of the features on a given part design Features that possess the same machining direction that can be machined on an assigned machine per the process sequence pro vided are grouped together and form one setup For the part design shown in Figure 5 both of the fea tures can be machined in the same setup The last activity of A2 see Figure is to identi fy potential reference planes A24 After determin ing the various machining setups the fixture design er attempts to identify reference planes or features for each setup The guiding theme in this activity is to accurately locate various features to be machined in a setup In general fixture designers prefer the use of pre machined or pre existing through holes and a pair of adjacent perpendicular faces Dowel pins inserted into through holes position the work piece accurately and hold that position well during machining A pair of flat plate locators butting against a corresponding pair of right angled surfaces rigidly ensures accurate work piece location during machining For the features to be machined in Figure 5 faces 1 5 and 7 can serve as potential ref erence planes Decomposition of Activity A3 Perform skeletal fixture design The activity performing a skeletal fixture design activity A3 in Figure 3 can be decomposed as shown in Figure 8 The term skeletal fixture design refers to the general design idea to support locate and clamp a workpiece in a setup For instance a fixture designer 64 Journal of Manufacturing System Vol 21 No 1 2002 Ml Figure 8 Decomposition of Perform Skeletal Fixture Design A3 may initially decide on 1 using a support against the bottom face of a part 2 using a pre machined hole to locate all features to be machined in that setup and 3 adopting a side clamping method to hold a workpiece This initial design is based on the fixture designer s understanding of the features to be produced the datum planes in that setup and the nature of the base and side faces that is are they planar and do they pos sess enough area for a clamp to be used among other issues As mentioned earlier experienced fixture designers usually focus on the detailed fixture design without developing a skeletal fixture design The domain experts opined that during their initial years they performed skeletal fixture design before detailing the design however as their experience increased they became more skilled at producing a feasible fix ture during t